The present invention relates to a method of digital demodulation.
It applies particularly to a receiving device implementing a reception diversity technique.
Diversity techniques are well known in the field of digital transmission. Among these techniques, mention may be made of:
spatial diversity, which can be used especially in radio transmission when several receiving sensors are arranged at different sites; PA1 frequency diversity, when the same information is sent simultaneously on different frequencies; PA1 time diversity, in the case where the same information is repeated. PA1 the selection method, consisting simply in choosing the observation exhibiting the best signal/noise ratio; PA1 the method known as "equal gain combining", in which a decision is taken on the basis of the sum of the observations after they have been brought into phase; PA1 the method known as "maximum ratio combining", in which a decision is taken on the basis of the sum of the squares of the observations brought into phase and divided by the estimated power of the noise by which they are affected. This latter method supplies a maximum signal/noise ratio after recombination. PA1 estimation of first demodulation parameters at a first end of the segment; and PA1 calculation of first estimates of symbols of the frame on the basis of the first estimated demodulation parameters and of the signal segment covered from the first end to a second end, PA1 estimation of second demodulation parameters at the second end of the segment; and PA1 calculation of second estimates of symbols of the frame on the basis of the second estimated demodulation parameters and of the signal segment covered from the second end towards the first end.
These various diversity techniques can also be combined with each other. The benefit of these techniques is that they make it possible to improve binary error rates in the estimates produced by the receiving device. Against that, they generally have the drawback of requiring additional resources in terms of passband and/or of complexity of the transmitting and receiving devices.
Numerous methods exist for combining the multiple estimates obtained by the diversity receiver, among which mention may be made of:
It may be that the observations available are not subjected to interference effects (noise, channel interference) which are completely non-correlated (particularly as far as channel interference is concerned). In this case, the conventional recombination methods do not achieve the expected results. Moreover, in digital transmissions, all that counts is the likelihood of the observations and the decision-taking which stems therefrom, in the maximum likehood sense; this aspect does not emerge explicitly in conventional methods.
One aim of the present invention is to provide an alternative, based on maximum likelihood, to the conventional methods for recombinations of estimates affected by various types of interference.